Tuesday, February 26, 2008

Tapping My First Bottom Bracket

This is a follow-up post from here, where I outline the history of the bottom bracket. In that thrilling episode, Aaron Goss tapped my drive-side bottom bracket shell to Italian standard, as it had been English, but had been completely stripped. Luckily, Aaron suggested I not retap the non-drive-side, which was in fine shape.

However, I couldn't pass up the opportunity to try cutting threads myself, especially since I don't much care for the frame (unsatisfactory lateral rigidity in the rear triangle). Little to lose, lots to gain.

The tool I was using is described here. It is definitely not shop grade.

Well, there's really not lots to tell. I used lots of cutting oil, tried to keep the shavings out of the cutter, used smooth effort. The only problem is that the diagram that came with the tool showed the tool assembled without a spring applying tension for cutting (although the diagram for facing did show use of the spring). It only took me a few seconds to realize that that couldn't be right; without the spring, the cutter does not stay firmly against the shell, and flops around. Couldn't possibly make a straight set of threads that way, so I improvised a bit.

Nominal testing indicates that the threads I cut accept a cup, and hold it, as readily as the ones Aaron cut. However, the true test will be when I install an actual bottom bracket, tightened the cups to specified torque, and have ridden it for several hundred miles. I'll want to got retro, and use a new 3-piece Italian bottom bracket, so I can examine the bearings, cones, and races for wear:

Does the adjustable cup install straight and permit a normal adjustment of the bottom bracket for play and free rotation?

Does the adjustable cup stay threaded in?

After several hundred miles of riding, do the patterns of wear on the cup races and axle cones indicate skew or play?

I wouldn't be surprised to find that my threads seat the adjustable cup slightly askew. Right now, I wish I had a BTS-1 so I could feel what it feels like to cut with the industry standard tool. But that's beyond my budget for awhile.